TOKYO, March 18 (Reuters) - Japanese engineers toiled
frantically to avert a catastrophic release of radiation from a
crippled nuclear power plant north of Tokyo on Friday, but the
United States said it could take weeks to cool the facility's
overheating fuel rods.

Officials said they hoped to fix a power cable to at least
two of the six reactors in the hope of restarting water pumps
and were preparing to douse them in the afternoon with water
from fire trucks.

However, no one was holding out hope that the crisis --
about to enter its second week after last Friday's 9.0-magnitude
earthquake and tsunami -- could be overcome anytime soon.

Japan's nuclear agency spokesman conceded that a "Chernobyl
solution" of burying the reactors in sand and concrete was in
the back of the authorities' minds.

Millions in Tokyo remained indoors on Friday, fearing a
blast of radioactive material from the complex 240 km (150
miles) to the north, though prevailing winds would likely carry
contaminated smoke or steam away from the densely populated city
to dissipate over the Pacific Ocean.

Japan's nuclear disaster, the world's worst since Chernobyl
in Ukraine 25 years ago, has triggered alarm and reviews of
safety at atomic power plants around the globe.

U.S. President Barack Obama, who stressed the United States
did not expect harmful radiation to reach its shores, announced
that he had ordered a comprehensive review of domestic nuclear
plants and pledged Washington's support for Japan.

"In the coming days, we will continue to do everything we
can to ensure the safety of American citizens and the security
of our sources of energy," he said. "And we will stand with the
people of Japan as they contain this crisis, recover from this
hardship, and rebuild their great nation."

The Group of Seven rich nations, stepping in together to
calm global financial markets after a tumultuous week, agreed to
join in rare concerted intervention to restrain a soaring yen.

The United States' top nuclear regulator said it could take
weeks to reverse the overheating of fuel rods at the Fukushima
Daiichi plant.

"This is something that will take some time to work through,
possibly weeks, as you eventually remove the majority of the
heat from the reactors and then the spent-fuel pools," Nuclear
Regulatory Commission Chairman Gregory Jaczko told a news
conference at the White House.

Yukiya Amano, head of the Vienna-based International Atomic
Energy Agency (IAEA), was due back in his homeland later on
Friday with an international team of experts after earlier
complaining about a lack of information from Japan.

Graham Andrew, his senior aide, called the situation at the
plant "reasonably stable " but the government said
white smoke or steam was still rising from three reactors and
helicopters used to dump water on the plant had shown exposure
to small amounts of radiation.

"The situation remains very serious, but there has been no
significant worsening since yesterday," Andrew said.

The nuclear agency said the radiation level at the plant was
as high as 20 millisieverts per hour. The limit for the workers
was 100 per hour.

COOLING PUMPS MAY NOT WORK

Even if the engineers manage to connect the power at the
Fukushima Daiichi plant, it is not clear the pumps will work as
they may have been damaged in the earthquake or subsequent
explosions and there are real fears of the electricity shorting
and causing another explosion.

Nuclear agency spokesman Hidehiko Nishiyama said it was
unclear how effective spraying water on the reactors from
helicopters had been on Thursday, but the priority now was to
get water into the spent-fuel pools.

"We have to reduce the heat somehow and may use seawater,"
he told a news conference. "We need to get the reactors back
online as soon as possible and that's why we're trying to
restore power to them."

Jaczko said the cooling pool for spent-fuel rods at the
complex's reactor No.4 may have run dry and another was leaking.

DOLLAR GAINS AS FINANCIAL LEADERS INTERVENE

The U.S. dollar surged more than two yen to 81.80
after the G7's pledge to intervene, leaving behind a record low
of 76.25 hit on Thursday.

U.S. markets, which had tanked earlier in the week on the
back of the crisis, rebounded on Thursday but investors were not
convinced the advance would last.

The yen has seen steady buying since the earthquake, as
Japanese and international investors closed long positions in
higher-yielding, riskier assets such as the Australian dollar,
funded by cheap borrowing in the Japanese currency.

Expectations that Japanese insurers and companies would
repatriate billions of dollars in overseas funds to pay for a
reconstruction bill that is expected to be much costlier than
the one that followed the Kobe earthquake in 1995 also have
helped boost the yen.

RADIATION LEVELS IN TOKYO BARELY ABOVE AVERAGE

The government had warned Tokyo's 13 million residents on
Thursday to prepare for a possible large-scale blackout but
later said there was no need for one. Still, many firms
voluntarily reduced power, submerging parts of the usually
neon-lit city in darkness.

The U.S. embassy in Tokyo has urged citizens living within
80 km (50 miles) of the Daiichi plant to evacuate or remain
indoors "as a precaution", while Britain's foreign office urged
citizens "to consider leaving the area". Other nations have
urged nationals in Japan to leave the country or head south.

Japan's government has told everyone living within 20 km (12
miles) of the plant to evacuate, and advised people within 30 km
(18 miles) to stay indoors.

At its worst, radiation in Tokyo has reached 0.809
microsieverts per hour this week, 10 times below what a person
would receive if exposed to a dental x-ray. On Thursday,
radiation levels were barely above average.

The plight of hundreds of thousands left homeless by the
earthquake and tsunami worsened following a cold snap that
brought heavy snow to worst-affected areas.

Supplies of water and heating oil are low at evacuation
centres, where many survivors wait bundled in blankets.

About 30,000 households in the north were still
without electricity in near-freezing weather, Tohuku Electric
Power Co. said, and the government said at least 1.6 million
households lacked running water.